Instant Room-Temperature Gelation of Crude Oil by Chiral

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Instant room-temperature gelation of crude oil by chiral organogelators Changliang Ren, Grace Hwee Boon Ng, Hong Wu, Kiat-Hwa Chan, Jie Shen, Cathleen Teh, Jackie Y. Ying, and Huaqiang Zeng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01367 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 14, 2016

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Chemistry of Materials

Instant room-temperature gelation of crude oil by chiral organogelators Changliang Ren,† Grace Hwee Boon Ng,‡ Hong Wu,† Kiat-Hwa Chan,† Jie Shen,† Cathleen Teh,‡ Jackie Y. Ying,† and Huaqiang Zeng*,† †

Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669

‡

Institute of Molecular and Cell Biology, 61 Biopolis Drive, The Proteos, Singapore 138673

ABSTRACT: Large-scale treatment of oily water arising from frequent marine oil spills presents a major challenge to scientists and engineers. Although the recently emerged phase-selective organogelators (PSOG) do offer very best promises for oil spill treatment, there still exists a number of technical barriers to overcome collectively, including gelators’ high solubility, high gelling ability, general applicability toward crude oil of various types, rapid gelation with room temperature operation, low toxicity and low cost. Here, a denovo-designed unusually robust molecular gelling scaffold is used for facile construction of a PSOG library and for rapid identification of PSOGs with the most sought-after practical traits. The identified gelators concurrently overcome the existing technical hurdles, and for the first time enable instant room-temperature gelation of crude oil of various types in the presence of seawater. Remarkably, these excellent gelations were achieved with the use of only 0.058-0.18 liter of environmentally benign carrier solvents and 7-35 g of gelator per liter of crude oil. Significantly, two out of 20 gelators could further congeal crude oil in the powder form at room temperature, highlighting another excellent potential of the developed modularly tunable system in searching for more powerful powder-based gelators for oil spill treatment.

INTRODUCTION Including the monstrous BP oil spill of millions of barrels of crude oil in the Gulf of Mexico in 2010,1 large-scale marine oil spills have occurred more than 50 times and released > 5 million tons of crude oil into the ocean over the period of 1965-2010.2 The devastating and lasting damage caused by spilled oils to the ocean ecosystem and environment, as well as the huge socioeconomic losses all call for the urgent development of efficient and sustainable oil spill control technologies. Current oil spill control measures largely rely on in situ burning of oil or use of skimmers and booms,3 sorbents,4-7 dispersants,8,9 bioremediation.10 and solidifiers.11 None of these approaches, however, are very effective on a large scale either on their own or in combination, especially in rough water.11-15 Phase-selective organogelators derived from small organic molecules have recently emerged as candidates as “smart” oilscavenging materials.16-24 The majority of the hitherto developed gelators necessitate carrier solvents for their dissolution before being applied to gel organic liquids. For gelators under this category, technical challenges to overcome include gelators’ high solubility in environmentally benign solvents, high gelling ability, general applicability toward crude oil of various types, rapid gelation with room temperature operation, low toxicity to marine lives and low cost. In particular, high solubility in essentially nontoxic carrier solvents such as ethanol and ethyl acetate, which are thousands of times less toxic than crude oil, represents an unsolved issue. Large amounts of carrier solvent,17 or toxic carrier solvent21 or maintenance of the solution under hot conditions18,23 was needed for gelling distilled petroleum fractions (e.g, petrol

and diesel) whose gelling properties nevertheless are very different from crude oil of complex structures. For a recently elaborated gelator able to gel highly mobile benzene and one type of crude oil of unknown properties in the powder form,24 it is particularly disadvantaged by a long gelation time even under constant stirring as a result of its low diffusion/dissolution speed into oil. This makes “on-site” collection impossible and increases the risk of pollution by oil during and after gelation. A long gelation time additionally suggests that this powder-based gelator might be incapable of gelling highly viscous heavy crude oil. Prior to our current work, gelators able to instantly solidify crude oil, rather than distilled petroleum fractions, at room temperature still remain elusive. This slow progress with limited practical potential could be attributed to the fact that the de novo design of PSOGs, concurrently possessing all the aforementioned desirable practical traits, is an extremely difficult task: the three dimensional (3D) entangled gelling network stabilized by non-covalent forces cannot be precisely and predictably manipulated at the molecular level. Herein, we report a few low-cost low-toxicity gelators that for the first time allow for instant gelation of crude oils of widely ranging densities and viscosities from oily water into waterfloating stiff solid with room-temperature operations and with the use of minimum amounts of environmentally benign solvents. The identification of these desired gelators was achieved via a combinatorial screening of a carefully designed PSOG library, possessing an unusually robust molecular gelling scaffold that consistently delivered gelators with high gelling ability regardless of side chains. Such a robust gelling scaffold is rarely seen among the hitherto reported gelling systems of diverse types.25-28 Another

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great advantage of our system lies in its modularly tunable nature. It is this unique feature that leads to high tunablity in gelators’ solubility, eventually culminating in the discovery of commercially viable gelators that concurrently overcome all the major technical barriers including an unprecedentedly high solubility (500 mM in environmentally benign solvents) for largescale application in mitigating ecosystem damage and environmental pollution caused by oil spills. Significantly, two out of twenty gelators could also gel a light crude oil in the powder form at room temperature, albeit with a long gelation time of 48 h. This very first generation of powder-based gelators sets the groundwork for the evolution of more powerful next-generation gelators for instantly gelling crude oil at room temperature in the powder form for oil spill treatment.

RESULTS AND DISCUSSION Design and features of the gelator library. The PSOG library (F-AA-Cn, Figure 1), accessible in a single synthetic step from Fmoc-protected amino acids, was designed to incorporate three key features that define the overall molecular framework of the resultant gelators: (i) two secondary amide bonds carrying two pairs of carbonyl O-atoms and amide H-atoms to possibly produce, via intermolecular H-bonds, a one-dimensionally (1D) aligned H-bonded -sheet-like columnar structure, (ii) a rigid planar conjugated aromatic Fmoc group to enhance the stability of the H-bonded 1D structure via robust aromatic π-π stacking forces, and (iii) a defined chirality that might help to lock the molecular backbone into a conformation predisposed towards forming the H-bonded 1D structure with reduced entropic penalties. In addition, we envisioned that the stabilities of the Hbonded 1D structures and the resultant higher order fibrous structures (i.e. nanofibers), their corresponding gelling ability and solubility can be further tuned by systematically and combinatorially varying R1 and R2 groups. These constant and variable structural features would combine collectively to facilitate the formation of H-bonded 1D structures from which 1D nanofibers could be formed, which further associated with one another to create a 3D entangled nanofiber-based gelling network for extensive solvent entrapment and oil solidification. To test the effect of these three molecular features on the oil-gelling ability of the designed PSOG molecules, a total of 20 library compounds derived from 5 R1 and 4 R2 groups was prepared by coupling the corresponding Fmoc-protected amino acids with four types of alkyl amines that differed in the length of hydrophobic carbon chains. Two types of control compounds, i.e., F-Gly-C4 and FGly-C8 that both lacked a chiral center, as well as F-Pro-C4 that lacked both the chiral center and two H-bond donor/acceptor sites, were also prepared to substantiate the intended roles played by these two structural features. Crystal structures of F-Phe-C4 and F-Gly-C4. To elucidate the above possible roles played by Fmoc, H-bonds and the chiral center in the formation of fibrous structures, growth of single crystals for the 20 gelators and 3 non-gelators (F-Gly-C4, F-GlyC8 and F-Pro-C4) was attempted. A combinatorial screening of hundreds of crystal growth conditions finally yielded X-ray quality single crystals for F-Phe-C4 and F-Gly-C4, which were obtained after 2-3 weeks by slowly diffusing 2.0 mL of n-hexane into 1.0 mL of F-Phe-C4-containing chloroform solution or by

Figure 1. Structural features of the gelator library F-AA-Cn and three control compounds. a) The library is derived from Fmocprotected amino acids and designed to contain three constant (Fmoc, H-bonds and chiral center) and two variable (R1 and R2) structural features. b) The three control compounds were made to elucidate the important roles played by the constant structural features, i.e., H-bonds and chiral center, in oil gelation.

evaporating a F-Gly-C4-containing solvent mixture (acetonitrile: tetrahydrofuran = 3:1, v:v). In both structures, both Fmoc group and H-bonds provided the primary directional driving forces, instructing the molecules to stack along axis b to produce wellorganized H-bonded 1D columnar stacks as envisaged (Figure 2). Due to the chiral center, one key difference between the two structures was manifested in the relative orientation of the two amide groups. In F-Phe-C4, the two amide groups were aligned in parallel with each other, and both participated in forming the 1D columnar stacks. In F-Gly-C4, they were roughly perpendicular to each other with only one of them contributing to the columnar stacking. As a result, the inter-columnar packing in F-Phe-C4 was exclusively mediated by weak van der Waals (VDW) forces, which might work together with oil molecules to produce a 3D entangled fibrous gelling network in a hydrophobic oil environment, whereas that of F-Gly-C4 was additionally stabilized by extensive well-aligned intermolecular H-bonds to produce planar ensembles that were not easily interrupted by weak VDW forces from oil molecules, indicating the poor solubility or precipitate-forming nature of F-Gly-C4 in oil. Indeed, F-Gly-C4 is poorly soluble in petrol, diesel and crude oil.27 Exceedingly high and broad spectrum oil-gelling abilities. The intended oil-gelling abilities for the designed monopeptidebased gelators and non-gelators were then examined by the “stable to inversion” method (See the Supporting Information, SI) in eight oils, including four types of crude oil (Grissik, Arab Light, Arab Heavy and Ratawi; Table S1, SI). These four crude oils cover wider ranges in physical properties (density, sulfur content and viscosity) than the world’s most actively traded oils (West Texas Intermediate, Brent Crude, and Dubai Crude) and many others (Tables S2-S3), and should constitute a good representation of the


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a

b

F-Phe-C4

Fmoc group

Intra-columnar

F-Gly-C4

Fmoc group

stacking along axis b

Intra-columnar

Top view

stacking along axis b

Top view

Fmoc group

Fmoc group

Side view

Side view

Inter-columnar packing along axis a

Inter-columnar packing along axis a H-bonds

H-bonds

Top view of inter-columnar packing along axis b

H-bonds

H-bonds

Top view of inter-columnar packing along axis b

Figure 2. Crystal structures of gelator F-Phe-C4 and non-gelator F-Gly-C4. a) The existence of a chiral center in F-Phe-C4 induces the two amide bonds to be vertically aligned and in parallel with each other, enabling efficient H-bond-mediated columnar stacking that is further assisted by aromatic - forces among Fmoc groups. b) A lack of chiral center makes the two amide bonds in F-Gly-C4 roughly perpendicular to each other, with one participating in forming 1D columnar stacks and the other forming extensive intermolecular H-bonds that act collectively with numerous weak van der Waals forces to allow the 1D columns formed to tightly associate with one another. In contrast, the inter-columnar associations in F-Phe-C4 rely on only weak van der Waals (VDW) interactions, which can be easily disrupted by other hydrophobic molecules.

majority of the world’s actively traded oils. (Table S2). Except for some limited cases for which the gelators were soluble in oil (Table S3), these 20 gelators could gel all eight oils with minimum gelation concentrations (MGCs in % w/v, mg/100 µL) ranging from 0.15 to 0.53% w/v for petrol, 0.11 to 0.32% w/v for diesel, 0.06 to 0.28% w/v for mineral oil, 0.08 to 1.85% w/v for silicone oil, 0.29 to 0.75% w/v for Grissik, 0.32 to 1.65% w/v for Arab Light, 0.75 to 3.10% w/v for Arab Heavy, and 0.62 to 1.65% w/v for Gratawi. All the gels were thermoreversible, easily filterable/scoopable, and stable for several months at room temperature, indicating their excellent temporal stability. The solid-state morphologies of the as-formed gels by the 15 gelators in petrol were visualized via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 3a and Figures S1-S4 illustrate the extensive formation of nanofibers of

40 to 800 nm in diameter, which intertwine to produce a 3D entangled network able to trap oil molecules and form the gels through surface tension and capillary forces. Gelators able to immobilize solvents to  100 times their own weight are defined as supergelators. By this conventional criteria, all the 20 gelators acted as supergelators for these oils, except for three cases (F-Leu-C8 and F-Phe-C6 with silicone oil, and FPhe-C8 for Grissik). For instance, F-Val-C8 gelled mineral oil with a MGC value of 0.06% w/v (= 0.06 mg/100 µl of mineral oil), which corresponds to an ability to immobilize mineral oil  1200 times its own weight, ranking among the best organogelators ever reported. The denser and more viscous Arab Light, Arab Heavy and Gratawi were more difficult to solidify with average MGC values of 0.95% w/v, 1.50% w/v and 1.12% w/v, respectively. In comparison, the three control compounds, F-Gly-



a

b

F-Ala-C4

F-Val-C4

F-Phe-C4

c Oil gelation

24 mm

Oil gelation and reclamation

5 mm

d

Vacuum distillation

Scooped out

105

G' G''

105

G', G'' (Pa)

G', G'' (Pa)

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104

103 0.1

1



10 (rad/s)

100

104 103

G' G''

102 101

10

0 (Pa)

100

Figure 3. Phase-selective gelation of oils, physical characterizations of gels and diesel recovery from solidified diesel gels. a) SEM (top) and TEM (bottom) images of gelled petrol via the “heating and cooling” method in the presence of F-Ala-C4, F-Val-C4 and F-Phe-C4. Red scale bars = 1 μm. b) Phase-selective gelation of petrol by F-Ala-C6 (1.4% w/v), diesel by F-Leu-C4 (1.6% w/v), Grissik by F-Ala-C4 (1.0% w/v), Arab Light by F-Leu-C4 (1.8% w/v), Arab Heavy by F-Leu-C4 (2.8% w/v) and Ratawi by F-Leu-C4 (3.0% w/v) in the presence of seawater at room temperature. Both diesel and petrol were dyed with 0.01% red Sudan III for clarity in visualization. The vertically aligned gels (24 mm diameter and 5-7 mm thick) shown at the bottom were prepared from the larger vials. c) Phase-selective gelation of bulk diesel in the presence of seawater, and its quantitative recovery through vacuum distillation. d) Dynamic rheological studies of Arab Heavy in phaseselective gelation by F-Leu-C4 (2.5% w/v). G’ = storage modulus, G” = loss modulus, frequency sweep = ω, and stress amplitude = 0.

C4, F-Gly-C8 and F-Pro-C4 could not gel any of the eight oils examined, demonstrating a necessity to have both the chiral center and H-bonds in the molecular backbone of gelator molecules. Room-temperature phase-selective gelation of crude oil. The possibility of using these gelators to selectively gel oil from oily water was explored in the presence of seawater. As the typical “stable to inversion”-based method required heating to dissolve the gelators in the liquid to be gelled, which would be impractical for the treatment of large-area oil spills, an alternative roomtemperature gelation protocol was examined. After screening all the routinely used organic solvents and their various combinations, we found that 4 out of the 20 gelators, F-Ala-C4, F-Ala-C6, F-Leu-C4 and F-Leu-C6, were sufficiently soluble in

solvent mixtures containing ethyl acetate/ethanol or acetone/ethanol. Accordingly, a high concentration of the gelator, e.g. 200 mg/mL of F-Leu-C4, was prepared in a solvent mixture containing ethyl acetate and ethanol (3:2, v:v), an aliquot of which (0.04 mL) was then added to a biphasic system containing 0.5 mL of oil, such as diesel, and 2 mL of seawater. Upon gentle shaking to simulate the choppy wave motion, spontaneous partitioning of the gelator into diesel and selective gelation of the oil phase occurred instantaneously, while leaving the aqueous layer intact. In less than 90 sec, diesel was fully gelled and stayed afloat on the seawater surface (Figure 3b). By this protocol, the corresponding biphasic MGC (BMGC in % w/v, mg/100 µL) value for F-LeuC4 against diesel was determined to be 1.2 % w/v, indicating that F-Leu-C4 was able to solidify diesel at 69 times its own weight.


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The full testing of all the 4 gelators against the 6 representative oils (diesel, petrol, Grissik, Arab Light, Arab Heavy and Ratawi) was then conducted. With the exception of F-Leu-C4 and F-LeuC6 against petrol, all the other 22 combinations yielded stable phase-selective water-floating gels within 28-97 sec (more vigorous shaking of oily water after addition of gelator solution significantly shortened the gelation time to within 6-28 sec for all cases) and excellent BMGC values of 0.7-2.8% w/v (Table S4). These BMGC values corresponded to oil:gelator mass ratios of 32.5-107. The longest gelation time of 97 sec was observed for FLeu-C4 against Ratawi. Generally, crude oil with higher density and viscosity requires more gelator to attain full gelation. The robustness of the phenomenon can be established by the fact that both the phase-selective gelation and the BMGC value remain minimally affected by the type of water (e.g. plain water, ground water and seawater) and pH (0 to 14). The gels’ high thermal and temporal stabilities were demonstrated by the fact that these four gelators have thermal decomposition temperatures of 171-190 C (Figure S5) and that the 18 gels exhibited a Tgel-sol value of ≥ 44 °C and remained stable at room temperature for more than 6 months. Our additional testing of all 20 gelators showed that, under very vigorous shaking, both F-Leu-C6 and F-Leu-C8 in the powder form can fully gel Grissik at room temperature after 48 h at 2.5% w/v. They, however, cannot congeal the other three types of crude oils (e.g., Arab Light, Arab Heavy and Ratawi). The above BMGC values show that, to gel one volume of weathered or unweathered crude oil, only 0.058-0.18 volume of solvents of extremely low toxicity was needed for dissolution of the required amount of gelators. This was much lower when compared to two reported examples, whereby 0.8 volume of ethanol or 0.33 volume of toxic hot petrol were needed to dissolve gelators for solidifying one volume of diesel17 or crude oil,20 respectively. For additional comparisons, other gelators able to gel one or a few refined oils (petrol, diesel, etc) are disadvantaged by their inability to gel some refined oils19,22 or a need to use toxic or hot co-solvent18,21,23 for dissolution of gelators. Very recently, Sureshan and his co-workers reported a sugarbased gelator able to gel crude oil in the powder form.24 Compared to the existing solution-based method, powder-based gelation is more advantageous in that no carrier solvents are needed and represents a significant advance in the field. Nevertheless, gelators of this type in general are greatly disadvantaged by (1) difficulty to treat large-scale marine oil spill as powders cannot be evenly sprayed from aircraft over large sea surface and (2) low diffusion/dissolution speed into oil (it took, under constant stirring, about an hour alone for the reported gelator to fully gel highly mobile benzene with a low viscosity of 0.6 mPas at 25 C). Such a long gelation time makes “on-site” collection impossible, thereby increasing the risk of pollution by the oil during and after gelation. In analogue to solidifiers generally suffering from an inability to satisfactorily solidify viscous medium or heavy crude oil and in view of the fact that only one type of crude oil of unknown property has been tested with no mentioning of exact gelation time, the powder-based approach might not be sufficiently general toward gelling various kinds of weathered and unweathered crude oils, particularly heavy crude oils (e.g., Arab Heavy and Ratawi). Our present investigation lends support to this perspective, suggesting both powder form and solution method have its own pros and cons, and neither is significantly better than the other. For the powder-based

gelators, the major challenge is to shorten gelation time needed for light, rather than heavy, crude oil to within minutes such that onsite collection becomes possible to prevent secondary pollutions resulting from treated oil. Oil weathering on the sea surface begins as soon as oil is spilled. To test if the gelators can achieve similarly efficient gelation of weathered oil, crude oil (4 mL each) was placed in an open vial of 22 mL in the presence of 2 mL seawater, left in fume hood with an air flow speed of ~ 0.5 m/s at room temperature for three weeks, and further subjected to oxidation via direct exposure to the tropical sunlight in Singapore at room temperatures of 2631 °C for three days. Despite weight losses of 7-62% and huge increases in viscosity across the four crude oils (Table 1), the determined BMGC values using gelator F-Leu-C4 confirm its outstanding ability to achieve instant room-temperature gelation of both weathered and unweathered crude oils. Table 1. Changes in physical properties of weathered crude oils and BMGC values determined using F-Leu-C4.a Crude Oil

Weight Loss (%)

Density (kg/L)

Viscosity at 25 oC (mPas)

BMGC (% w/v)

Grissik

62

0.77 (0.75)

2.2 (0.66)

1.6 (1.3)

Arab Light

32

0.85 (0.83)

12.2 (2.7)

2.3 (1.4)

Arab Heavy

12

0.92 (0.89)

171.3 (42.5)

3.2 (2.5)

Ratawi

7

0.93 (0.91)

330.5 (81.9)

3.5 (2.8)

a

200 mg/mL of F-Leu-C4 in ethyl acetate and ethanol (3:2, v:v). The values shown in bracket are for unweathered oils.

The ease of removal and reclamation of treated oils is an important practical aspect that would help to permanently eliminate possible damage of the spilled oil to the chemical, physical and biological integrity of the ecosystem. Firstly, a biphasic solution containing Ratawi (2.00 g) and water (10.00 g) was prepared to which 0.09 g of gelator dissolved in 0.18 mL (0.14 g) of ethanol and 0.27 mL (0.25 g) ethyl acetate (2:3, v:v) was added, and the resultant solution was vigorously shaked for 20 sec. Assuming water-immiscible ethyl acetate and gelator molecules both remain 100% in gelled oil, the gelled oil should weight 2.34 g. After filtration and from the weight of ethanolcontaining water, total mass of collected solidified oil containing gelator and ethyl acetate was calculated to be 2.44 g, which is 0.10 g more than the theoretically allowed value. Visual inspection of the water solution confirms no visible oil droplets or oil slick remains in water, thereby confirming that majority of crude oil has been separated from water upon solidification. Secondly, to examine reclamation of treated oil, gelation experiment in a larger scale was conducted using 120 mg of F-Leu-C4, 10 mL of diesel and 30 mL of seawater (Figure 3c). The fully solidified diesel, which was stiff enough to be easily scooped out using a spatula, was transferred to a round-bottom flask and subjected to vacuum distillation, which resulted in a quantitative recovery of the diesel. Although the gelators could be recycled and re-used, the recyclability was limited to light oils (petrol or diesel), and not heavy crude oil containing components of high boiling points. Real world applications further require the floating gels to be robust enough under the choppy wave motion in rough seawater and to allow for easy collection and separation. In this regard, the



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Intercolumnar packing

R1  H

Intercolumnar packing

De/re-packing by oil

Fibers

De/re-packing by oil Inter-columnar assemblies via VDW forces

Inter-columnar packing

R1 = H

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intertwining

Fibers with a critical mass (40-800 nm in diameter)

Gelling network

Inter-planar packing

via H-bonds

(F-Gly-C4)

Columnar stacking

H-bonded 2D planar assemblies of columnar stacks

Insoluble 3D ensembles (non-gelling networks)

Figure 4. Plausible hierarchies of supramolecular connectivities underscoring a different gelling ability between gelators and non-gelling glycine derivatives (F-Gly-C4 and F-Gly-C8). In a hydrophobic environment, aromatic π-π stacking, H-bonding and extensive inter-planar VDW interactions are not easily disrupted by weak VDW forces. Columnar stacks, H-bonded 2D planar assemblies and 3D ensembles therefore could be readily formed in solution. Oil molecules, however, do exert variable influences on the sizes and structures of the intercolumnar assemblies facilitated by VDW forces. Since F-Gly-C4 and F-Gly-C8 both precipitate out from oil upon cooling, these extensively aggregated 3D ensembles, which are assumed to prevail in solution via strong inter-planar VDW forces, should be oil-insoluble, too.

mechanical strength of the organogel formed phase-selectively from oily water using F-Leu-C4 at its BMGC value was examined in rheological studies at 25 oC. In all four frequency sweep experiments (Figure 3d and Figure S6), the storage modulus G’ was essentially independent of frequency and much higher than the viscous modulus G” over the frequency range, suggesting excellent elasticity of the gels, which would not relax over long time scales. The high G’ values (5.0×104, 7.4×104, 5.5×104 and 1.4×105 Pa for diesel, Arab light, Arab heavy and Ratawi gels, respectively) pointed to the remarkable stiffness and strength of these gels (Figure 3b). The stress amplitude (0) at which a sharp decrease in moduli occurred corresponded to the yield stress of the gel, and high values of >80 Pa were obtained, demonstrating the ability of these gels to withstand high pressures, i.e. going well beyond supporting their own weight in an inverted vial. Toxicity of oils and gelators. The OECD approved protocol (OECD test guideline 236)29 with validated intra- and interlaboratory reproducibility30 was followed to test the aquatic safety of oils and gelator F-Leu-C4 using freshly fertilized zebrafish embryos and 3-day-old larvae as the test organisms over a time period of 96 h. The data showed the four crude oils to have a high toxicity, causing ~30% death of zebrafish larvae at a concentration as low as 2 ppm after three days, while F-Leu-C4 and F-Leu-C6 produced no harmful effects toward both zebrafish embryo and larvae after four days at concentrations of up to 100 ppm (Table S7 and Figures S7-S12). Surprisingly, structurally similar alanine derivatives were much more toxic with 97% death of zebrafish larvae caused by F-Ala-C4 after 2 days at 12.5 ppm and 22% death by F-Ala-C6 after 4 days at 50 ppm (Figure S13). Mechanistic insights into gelling and non-gelling networks. To assess whether the three structural features (Fmoc, H-bonds and a chiral center) designed into the molecular backbone of the

gelators work in concert with the expected roles (Figure 1), physical characterizations were conducted in the solution and solid state. Upon dilution from 100 mM to 0.8 mM in CDCl3, which allowed hydrogen-bonding, concentration-dependent 1H-NMR study at 25 C revealed significant up-field shifts of the amide Hatoms Ha and Hb in F-Ala-C4 by 0.25 ppm and 0.35 ppm (Figure S14a), respectively, indicating the involvement of Ha and Hb in forming intermolecular H-bonds. Similar concentration-dependent up-field shifts were also observed for gelators F-Ala-C6, F-LeuC4 and F-Leu-C6 and non-gelator F-Gly-C4, which consisted of two secondary amide bonds, but not for F-Pro-C4, which has only one such group (Figures S14b-c and S15). These results demonstrated that, except for F-Pro-C4, all the (non)gelator molecules carrying two secondary amide groups were capable of self-assembling via intermolecular H-bonds to produce extended H-bonded supramolecular structures. Intermolecular associations among Fmoc groups were also detected in fluorescence emission spectra at various concentrations in a solvent mixture of chloroform and n-hexane (1:4, v:v). With an increase in concentration from 0.1 mM to 1 mM and to the gel-forming 10 mM (0.37% w/v), the emission maximum of F-Ala-C4 was red shifted by 10 nm (Figure S14a), which was indicative of aromatic π-π stacking among the Fmoc groups.32 The fact that such a red shift (9-16 nm) persisted with all the molecules including F-ProC4 (Figures S14 and S16) firmly established the Fmoc group as a robust π-π stacking moiety that contributed reliably to the formation of oil-gelling fibrous structures. Circular dichroism (CD) experiments (Figures S14 and S17) confirmed chirality in the resultant H-bonded supramolecular structures, which presumably was imparted by the chiral center in the gelator molecules such as F-Ala-C4. For F-Phe-C4, further analyses of X-ray diffraction (XRD) patterns of its crystal, fibrous gelling


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aggregates, and solid powder revealed a significant similarity between the crystal structure and the fibers (Figure S18), suggesting the ordered 1D columns of varying lengths, the basic units of the nanofibers, resembled those seen in the crystal structure. The above findings, combined with the crystal structures presented in Figure 2, led us to propose two hierarchies of supramolecular connectivities to account for the differences in gelling abilities (Figure 4). For gelators containing a chiral center, both the two amide groups participated in forming the columnar stacks, which underwent further self-associations to create intercolumnar assemblies of varying structures and sizes via VDW forces. In a hydrophobic environment, the oil molecules might play an important role in de/re-packing these inter-columnar assemblies. Once these assemblies reached a critical mass with a diameter of 40-800 nm, they would intertwine as a 3D entangled fibrous gelling network. As for the non-gelling precipitateforming glycine derivatives (F-Gly-C4 and F-Gly-C8) that lacked a chiral center, only one amide group was involved in forming Hbonded 1D columns with the other cross-linking numerous 1D columns into 2D planar assemblies of varying sizes, giving rise subsequently to the 3D ensembles stabilized by extensive interplanar VDW forces under little influence from the oil molecules. These extensively aggregated 3D ensembles were expectedly oilinsoluble, and hence incapable of producing gelling aggregates.27

CONCLUSION Our current work demonstrated a robust and modularly tunable molecular gelling scaffold for reliably and combinatorially constructing organogelators with high gelling ability while allowing their solubility in environmentally benign solvents to be highly tunable. Out of 20 library compounds with high gelling ability, only two (e.g., F-Leu-C4 and F-Leu-C6) were found to concurrently possess excellent solubility, likely non-toxicity to marine lives and unprecedented ability to instantly solidify both weathered and unweathered crude oils in the presence of seawater at room temperature. This, together with inexpensive material costs ($0.6 - $2.5 for leucine derivatives vs current cost of ~$100 for treating per liter of crude oil31) and excellent prospect of eliminating secondary pollutions from treated oil via onsite removal and easy recovery, makes the gelators practical and commercially viable for cleaning up oil spills, while ensuring environmental safety and ecosystem integrity. The fact that both F-Leu-C4 and F-Leu-C6 could also gel light crude oil in the powder form points to a high likelihood of finding more powerful powder-based gelators for their eventual uses in oil spill treatment.

ASSOCIATED CONTENT Supporting Information. Synthetic procedures for 20 gelators as well as a full set of characterization data including 1H NMR, 13 C NMR, (HR)MS, X-ray crystal file/data sheet for F-Gly-C4 and F-Phe-C4, minimum gelation concentration data, SEM and TEM images, TGA analysis curves, rheological data, toxicity data, 1H NMR dilution experiments, fluorescence spectra, circular

dichroism spectra and powder X-ray diffraction spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the Institute of Bioengineering and Nanotechnology (Biomedical Research Council, Agency for Science, Technology and Research, Singapore).

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TOC graphic

Room Temperature Operation


Instant Room-Temperature Gelation of Crude Oil by Chiral

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